Open AccessAdditive effects of acidification and mineralogy on calcium isotopes in Triassic/Jurassic boundary limestones
Author(s) -
Jost Adam B.,
Bachan Aviv,
van de Schootbrugge Bas,
Brown Shaun T.,
DePaolo Donald J.,
Payne Jonathan L.
Publication year - 2017
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1002/2016gc006724
Subject(s) - geology , aragonite , diagenesis , ocean acidification , excursion , seawater , extinction event , isotopes of carbon , calcite , carbon cycle , δ13c , carbonate , paleontology , horizon , total organic carbon , mineralogy , oceanography , stable isotope ratio , chemistry , environmental chemistry , population , ecosystem , law , biological dispersal , ecology , sociology , biology , quantum mechanics , political science , physics , demography , organic chemistry , astronomy
The end‐Triassic mass extinction coincided with a negative δ 13 C excursion, consistent with release of 13 C‐depleted CO 2 from the Central Atlantic Magmatic Province. However, the amount of carbon released and its effects on ocean chemistry are poorly constrained. The coupled nature of the carbon and calcium cycles allows calcium isotopes to be used for constraining carbon cycle dynamics and vice versa. We present a high‐resolution calcium isotope ( δ 44/40 Ca) record from 100 m of marine limestone spanning the Triassic/Jurassic boundary in two stratigraphic sections from northern Italy. Immediately above the extinction horizon and the associated negative excursion in δ 13 C, δ 44/40 Ca decreases by ∼0.8‰ in 20 m of section and then recovers to preexcursion values. Coupled numerical models of the geological carbon and calcium cycles demonstrate that this δ 44/40 Ca excursion is too large to be explained by changes to seawater δ 44/40 Ca alone, regardless of CO 2 injection volume and duration. Less than 20% of the δ 44/40 Ca excursion can be attributed to acidification. The remaining 80% likely reflects a higher proportion of aragonite in the original sediment, based largely on high concentrations of Sr in the samples. Our study demonstrates that coupled models of the carbon and calcium cycles have the potential to help distinguish contributions of primary seawater isotopic changes from local or diagenetic effects on the δ 44/40 Ca of carbonate sediments. Differentiating between these effects is critical for constraining the impact of ocean acidification during the end‐Triassic mass extinction, as well as for interpreting other environmental events in the geologic past.